Polyolefins are widely used commercially because of their robust physical properties. For example, various types of polyethylenes, including high density, low density, and linear low density polyethylenes, are some of the most commercially useful. Polyolefins are typically prepared with a catalyst that polymerizes olefin monomers.
Low density polyethylene is generally prepared at high pressure using free radical initiators or in gas phase processes using Ziegler-Natta or vanadium catalysts. Low density polyethylene typically has a density at about 0.916 g/cm3. Typical low density polyethylene produced using free radical initiators is known in the industry as “LDPE”. LDPE is also known as “branched” or “heterogeneously branched” polyethylene because of the relatively large number of long chain branches extending from the main polymer backbone. Polyethylene with a similar density that does not contain branching is known as “linear low density polyethylene” (“LLDPE”) and is typically produced with conventional Ziegler-Natta catalysts or with metallocene catalysts. “Linear” means that the polyethylene has few, if any, long chain branches, typically referred to as a g′VIS value of 0.97 or above, such as 0.98 or above. Polyethylenes having still greater density are the high density polyethylenes (“HDPEs”), e.g., polyethylenes having densities greater than 0.940 g/cm3 and are generally prepared with Ziegler-Natta or chrome catalysts. Very low density polyethylenes (“VLDPEs”) can be produced by a number of different processes yielding polyethylenes typically having a density 0.890 to 0.915 g/cm3.
Copolymers of polyolefins, such as polyethylene, have a comonomer, such as hexene, incorporated into the polyethylene backbone. These copolymers provide varying physical properties compared to polyethylene alone and are typically produced in a low pressure reactor, utilizing, for example, solution, slurry, or gas phase polymerization processes. Polymerization may take place in the presence of catalyst systems such as those employing a Ziegler-Natta catalyst, a chromium based catalyst, or a metallocene catalyst. The comonomer content of a polyolefin (e.g., wt % of comonomer incorporated into a polyolefin backbone) influences the properties of the polyolefin (and composition of the copolymers) and is dependent on the identity of the polymerization catalyst. As used herein, “low comonomer content” is defined as a polyolefin having less than about 8 wt % of comonomer based upon the total weight of the polyolefin. The high molecular weight fraction produced by the second catalyst compound may have a high comonomer content. As used herein, “high comonomer content” is defined as a polyolefin having greater than about 8 wt % of comonomer based upon the total weight of the polyolefin.
A copolymer composition, such as a resin, has a composition distribution, which refers to the distribution of comonomer that forms short chain branches along the copolymer backbone. When the amount of short chain branches varies among the copolymer molecules, the composition is said to have a “broad” composition distribution. When the amount of comonomer per 1000 carbons is similar among the copolymer molecules of different chain lengths, the composition distribution is said to be “narrow”.
Like comonomer content, the composition distribution influences the properties of a copolymer composition, for example, stiffness, toughness, environmental stress crack resistance, and heat sealing, among other properties. The composition distribution of a polyolefin composition may be readily measured by, for example, Temperature Rising Elution Fractionation (TREF) or Crystallization Analysis Fractionation (CRYSTAF).
Also like comonomer content, a composition distribution of a copolymer composition is dependent on the identity of the catalyst used to form the polyolefins of the composition. Ziegler-Natta catalysts and chromium based catalysts produce compositions with broad composition distributions (BCD), whereas metallocene catalysts typically produce compositions with narrow composition distributions (NCD).
Furthermore, polyolefins, such as polyethylene, which have high molecular weight, generally have desirable mechanical properties over their lower molecular weight counterparts. However, high molecular weight polyolefins can be difficult to process and can be costly to produce. Polyolefin compositions having a bimodal molecular weight distribution are desirable because they can combine the advantageous mechanical properties of a high molecular weight (“HMW”) fraction of the composition with the improved processing properties of a low molecular weight (“LMW”) fraction of the composition. As used herein, “high molecular weight” is defined as a number average molecular weight (Mn) value of 100,000 or more. “Low molecular weight” is defined as an Mn value of less than 100,000.
For example, useful bimodal polyolefin compositions include a first polyolefin having low molecular weight and low comonomer content while a second polyolefin has a high molecular weight and high comonomer content. Compositions having this broad orthogonal composition distribution (BOCD) in which the comonomer is incorporated predominantly in the high molecular weight chains can provide improved physical properties, for example toughness properties and environmental stress crack resistance (ESCR).
There are several methods for producing bimodal or broad molecular weight distribution polyolefins, e.g., melt blending, reactors in series or parallel configuration, or single reactor with bimetallic catalysts. However, these methods, such as melt blending, suffer from the disadvantages brought by the need for complete homogenization of polyolefin compositions and high cost.
There is a need in the art for linear polyolefin copolymers having a high comonomer content and high molecular weight. There is also a need for BOCD polyolefin copolymer compositions having increased density split and high comonomer content.